This invention relates to coextrusion, particularly of a composite having thermally distinct layers.
Composites including layers having thermally distinct processing requirements, are of considerable commercial importance. In the case of some materials, thermal sensitivity is a problem during processing. For example, material degradation and equipment fouling may result. In other cases, it is beneficial to take advantage of thermal sensitivity, by for instance providing useful surface effects. For example, a surface oxidation-promoting, melt temperature benefits surface adhesion of materials such as polyethylene (PE). In even other cases, high temperatures are necessary for processability due to for instance, high melt viscosity.
One such composite is a liquid packaging container including a paperboard/low density polyethylene/low density polyethylene structure. The challenge is to provide a paperboard-contacting, PE layer with a preferred surface oxidation-promoting, melt temperature of about 315.degree. C. or more for good surface bonding to the paperboard, and to provide a liquid-contacting, PE layer with a preferred lower melt temperature of approximately 275.degree. C. to 285.degree. C. The higher melt temperature promotes bonding by promoting oxidation of the PE surface and enhancement of surface wetting. The lower melt temperature reduces surface oxidation, and provides improved heat seal performance and improved organoleptic properties including decreased residual odor.
Another such composite includes an oriented polypropylene substrate/low density polyethylene/ethylene vinyl acetate structure. The PE layer benefits from a melt processing temperature of about 300.degree. C. or more for promotion of substrate adhesion, whereas the ethylene vinyl acetate (EVA) layer preferably has a melt processing temperature of about 230.degree. C. or less to minimize residual odor. Also, EVA is thermally sensitive at elevated temperatures, with decomposition being further accelerated by metal contact.
Also including layers having thermally distinct processing requirements, is protective document laminating film including an oriented polyester substrate/polyethylene/ethylene vinyl acetate structure. EVA is a desirable thermoplastic for document contact because of its low heat sealing temperature. However, as may be understood from the previous example, PE and EVA typically have thermally distinct processing requirements.
As can be understood by one skilled in the art, composites including layers having thermally distinct processing requirements, have application not only to extrusion coating but also to extrusion lamination and to cast film and sheet products. These composites include polyamide and ethylene vinyl alcohol (EVOH) layered structures. EVOH is a thermally sensitive polymer with a tendency to crosslink, accelerated by elevated temperature and metal contact, as a result of which degradation and contamination typically result. Thus, a preferred melt processing temperature for EVOH is about 220.degree. to 230.degree. C. By comparison, polyamide-6 has a preferred melt processing temperature of about 270.degree. C., and becomes effectively non-processable at temperatures of about 250.degree. C. due to its extremely high flow viscosity.
A common solution for coating a substrate with layers having thermally distinct processing requirements, uses extrusion stations that operate at different processing temperatures. One such approach is exemplified in FIG. 8 of U.S. Pat. No. 4,701,360 to Gibbons et al, which describes coating a paperboard substrate with PE and thereafter coating the exposed PE surface with EVOH. This approach is generally illustrated in FIG. 1, which shows a substrate 1 being passed through a first extrusion coating station 2, at which a high melt temperature extrudate 3 is applied, quenched and solidified, and then being passed through a second coating station 4, at which a low melt temperature extrudate 5 is applied, quenched and solidified onto the extrudate/substrate composite. Similar is the process illustrated in FIGS. 9 and 10 of U.S. Pat. No. Re. 33,376 to Gibbons et al. An economic disadvantage is that this solution uses more than one extrusion station.
Another solution is to extrude the melt streams as exemplified by U.S. Pat. No. 3,797,987 to Marion. In this approach, streams of substantially different temperature are fed into a dual outlet, extrusion apparatus. The apparatus includes two separate flow channels each with a transverse flow-providing manifold and outlet; a common thermal barrier between and in fluid contact with the flow streams; and thermally independent, body halves separated by the thermal barrier. Disadvantages include the potential for entrapment of volatiles within the extruded composite, with negative affect upon organoleptics; metal surface contact of each stream face during the entire residence within the extrusion apparatus; and the potential for air entrapment between the melt streams as they are melt laminated together at the coating nip. Furthermore, as in the case of the FIG. 1 solution, extruded composite thickness, cross machine thickness variation and nominal individual layer thickness may be greater and hence manufacturing costs may be more than if the layers were coextruded. For purposes of this invention, the term "coextruded" is meant to require flow stream convergence to form a composite followed by transverse spreading of the composite.
A prior approach of the inventor has been to combine streams of substantially different temperatures in a conventional feedblock and to thereafter effect transverse spreading of the composite stream in a die body having thermally independent, body halves. However, this approach has not been entirely satisfactory. For example, the feedblock is set to a temperature between the temperatures of the streams, which significantly compromises thermal differentiation. Moreover, it would be difficult to produce a composite characterized by a small, yet distinct, temperature differential between layers or layer surfaces, from streams with a small temperature differential.
Therefore, there is a need for an improved extrusion process for providing composites including layers or layer surfaces having thermally distinct, processing needs. As can be understood from the foregoing discussion, by the term "thermally distinct" is generally meant a temperature difference of at least about 20.degree. C., more typically at least about 25.degree. to 30.degree. C. However, it will be appreciated by one skilled in the art that in the case of certain materials and processing, a processing difference of about 10.degree. C. will be thermally distinct.